The main objective of this application is to determine whether increased expression or change in the structures of three proto-oncogenes, c-mos, c- fos, and int-1, results in an increased incidence of tumors in mice. These inappropriate times and in inappropriate tissues affects normal development. One experiment focuses solely on the developmental consequences of introducing a point mutation into the purported serine/threonine kinase encoding domain of the c-mos gene. The approach that will be taken to introduce oncogenes having modified expression or structure into mice makes use of embryo-derive stem (ES) cells. These cells are transfected with plasmid constructs containing either or proto-oncogenes driven by an inducible promoter (structurally altered proto-oncogenes). Transfected ES cells are then microinjected in to blastocysts to generate chimeric mice. Since ES cells populate the germ line with good frequency, transgenic mouse lines having modified proto- oncogenes can be established. Two advantages of the ES cell/chimera system as compared to DNA microinjection to produce transgenics are the ability to characterize transfected ES cells prior to chimera production and the ability to target mutations by homologous recombination into endogenous genes of interest. The experiments described in this application make full use of this method of gene transfer to explore the potential role of proto- oncogenes in tumorigenesis as well as their functions during normal development. The proposed experiments address 4 specific questions: 1) Does the elevated expression of the proto-oncogenes c-mos, c-fos, and int-1 increase the incidence of tumors in mice and/or affect normal development adversely? 2) Are changes in the structure of the c-fos gene that have been demonstrated to be necessary for cell transformation in cultured cells also necessary for tumor formation in mice? 3) Does the join elevated expression of c-fos and c-mos in mice synergistically affect the incidence, location, and/or growth of tumors? 4) Does a histidine-190 to tyrosine substitution in c-mos protein result in loss of serine/threonine kinase activity and does this point mutation affects biological function?